X-Ray CT (computed tomography) apparatuses are apparatuses for calculating X-ray absorption coefficients at positions of an imaging object from measured projection data obtained by capturing images of the imaging object from many directions, and obtaining an image of X-ray absorption coefficient distribution (henceforth also referred to as CT image). By replacing the X-ray absorption coefficients with normalized CT values (−1000 for air, and 0 for water), a CT image is generated.
The CT image shows a tomographic image of a subject. On medical sites, pathological conditions of patients can be accurately and instantly diagnosed by using CT images, and therefore they are clinically useful. However, in order to obtain a CT image of high image quality required for diagnosis by medical practitioners, radiation exposure of a certain extent is unavoidable. If the irradiation dose is reduced in order to realize low exposure, ratio of noise to the detected signals increases, and there are thereby generated many line-shaped streak artifacts and grain-like noise, which cause incorrect diagnoses. Therefore, it has been desired to simultaneously realize both diagnosis of good quality and low radiation exposure by reducing the streak artifacts and noises at the time of low radiation dose imaging.
Patent document 1 discloses a method for generating a noise map in which image of imaging object has been removed (difference image). In this method, a plurality of measured projection data obtained by scanning the same positions of one imaging object at substantially the same times are divided into two sets of projection data according to projection angles, CT images are reconstructed from the sets of the divided projection data, respectively, a difference image of the two CT images is obtained, and a noise map is obtained from this difference image. According to the technique of Patent document 1, a filter is created by using this noise map to remove noise of the CT image. In this method, the filter is created on the basis of the noise map obtained from actual CT images, and therefore image of the imaging object is less easily degraded compared with the case of using a conventional filter for eliminating radio frequency components.